Treatment of polymer using silver

ABSTRACT

A method is provided by which a silver coating is applied to a polymeric substrate using an aqueous solution of silver nitrate. Also provided is a method for incorporation of silver as silver nitrate into a polymeric composition suitable for molding.

BACKGROUND OF THE INVENTION

The invention relates to the treatment of a material with silver. In oneaspect the invention relates to the coating of a polymeric material withsilver. In another aspect, the invention relates to the incorporation ofsilver in a moldable polymeric material.

It is known that a metal coating can be applied to the surface of asolid material. There are numerous utilities for the resultingmetal-laminated object, depending upon the metal and substrate used.Silver is one metal which is, for certain utilities, desirable to coaton a polymeric surface. A silver coating might, for example, be appliedto a polymeric surface to provide a laminate with an electricallyconductive layer. Conventional methods to coat a surface with silver,however, are generaly somewhat complicated and time-consuming.

It is an object of the present invention to provide a simple method forcoating a surface with silver. It is a further object to provide amethod for treating a polymeric material to apply a thin film of silveron the surface thereof. In another embodiment, it is an object of theinvention to provide a polymeric molding compound which has silverincorporated therein.

BRIEF SUMMARY OF THE INVENTION

According to the invention, a solid object is coated with silver by aprocess comprising contacting a wettable surface of the object with asolution of silver nitrate (AgNO₃) and heating the thus-contactedsurface to a temperature effective to deposit a silver film on theobject. The AgNO₃ is preferably in the form of an aqueous solution. Asilver coating can generally be established by heating the contactedsurface to a temperature of 350° to 500° F. Suitable substrate materialsinclude polyolefins and polymeric thermoplastics, includingpoly(phenylene sulfide).

In a second embodiment of the invention, AgNO₃ is physicallyincorporated in a polymeric material, and the resulting compound can befurther processed, such as by extrusion, to produce an article havingsilver dispersed throughout.

DETAILED DESCRIPTION OF THE INVENTION

Any suitable solid polymeric material can be coated with silveraccording to the invention method. Particularly suitable materialsinclude thermoplastic polymers, including polyethylene, polypropylene,poly(phenylene sulfide) and copolymers comprising these. Thermoplasticpolymers and copolymers of olefinic monomers are included within thescope of materials suitable for coating by the invention method.Polymers and copolymers comprising aromatic sulfides, such aspoly(arylene sulfide) are also suitable. A particularly preferredmaterial to coat according to the invention method has been found to bepoly(phenylene sulfide), a thermoplastic polymer having alternatingphenyl and sulfide groups.

AgNO₃ is used as the silver-depositing agent. Other nitrates such aszinc, copper, and tin nitrates, were tried using the invention methodand were found inferior to silver in depositing a metallic coating on apoly(phenylene sulfide) substrate. It is preferred that the silvernitrate be applied to the substrate in the form of an aqueous solution.The concentration of silver nitrate in the solution is not critical andcan be selected to produce the quality of coating desired. On a molarbasis, solutions in the range of about 0.005 to about 3 M are effectivein producing a silver-containing layer on the substrate. For mostpurposes, solutions having a AgNO₃ concentration in the range of about0.05 to about 2 M will be suitable.

The surface to be coated is contacted with the solution of AgNO₃. Themethod of contacting is not critical, and suitable methods includedipping, brushing, and spraying.

Optionally and preferably, the polymeric surface to be coated will bemade water-wettable, i.e. treated to render the surface more receptiveto the aqueous solution and more capable of overcoming the surfacetension of the aqueous solution. Methods such as flame treatment orcorona discharge can be used. One suitable flame treatment methodinvolves passing a 10% excess of air-to-methane oxidizing flame over thesurface for a time sufficient to alter the surface characteristics asconfirmed by reduction or elimination of beading of a water drop on thesurface.

The area to which the solution of AgNO₃ has been applied can then beheated, if necessary to a temperature effective for depositing a solidlayer of silver on the treated surface. The temperature used will dependupon the substrate being treated. For example, a suitable temperaturefor depositing silver onto a poly(phenylene sulfide) substrate willgenerally be at least about 350° F., and will generally range from about400° to about 700° F.

The treated surface is then permitted to cool to ambient temperature. Acoating of silver will adhere to the solid object, the thickness of thecoating depending generally upon the concentration of AgNO₃ used.

A thus-coated solid object can be used in applications requiring apolymeric material having an electrically conductive coating. Suchapplications include microcircuitry for printed circuit boards and radiofrequency shielding, in addition to decorative utilities.

According to the second embodiment, the silver can be incorporated inany suitable material requiring evenly-dispersed silver. Extrudablepolymeric materials are suitable. Preferred extrudable materials includepolyolefins and poly(arylene sulfide)s. Polyethylene, polypropylene andpoly(phenylene sulfide) are extrudable materials in which AgNO₃ can beincorporated to produce a silver-containing extruded article accordingto the invention.

A silver salt, such as AgNO₃, is physically blended within the polymericmaterial to be treated. Methods of such application include spraying,dipping or brushing a liquid solution of a silver salt onto solidpolymer in the form, for example, of pellets or incorporating a silversalt in a solid form in a finely-divided or liquefied polymeric resin.The polymeric resin is then molded into formed parts. This can beaccomplished by any suitable molding method, such as extrusion. Forexample, poly(phenylene sulfide) pellets in solid form can be dipped inor sprayed with an aqueous solution of AgNO₃ and then melt extruded intoformed parts. The resulting solid article contains dispersed silver andis characterized by a color change and a reduction in sulfurous odor.The thus-treated molded article can be used for any purpose requiring achemically-resistant plastic article, such as radio frequency shielding.

EXAMPLE

A series of experiments was done to demonstrate the operability of theinvention method of silver-coating a polymer.

A 1-g sample of silver nitrate salt was placed in a beaker. Distilledwater (3 cc) was added to the beaker with stirring to produce a creamywhite solution approximately 2M in silver nitrate.

A sample of Ryton® poly(phenylene sulfide), a product of PhillipsPetroleum Company, was flame treated with an oxygen-rich flame to renderthe surface water-wettable. On this surface was applied, using a cottonswab, the sodium nitrate solution in an "S" pattern.

An additional 3 cc of distilled water was added to the beaker solutionto produce a 1 M silver nitrate solution. This solution was also appliedto the flame-treated surface in an "S" pattern.

An additional 54 cc of distilled water was added to the beaker solutionresulting in an approximately 0.1 M solution. This solution was appliedto a second poly(phenylene sulfide) slab having the same characteristicsas the first.

An additional 640 cc of distilled water was added to the beaker solutionto produce an approximately 0.01 molar solution, which was applied tothe second poly(phenylene sulfide) slab.

After the samples were essentially dry, an oxygen-rich flame was passedover them. The 2M solution resulted in a slight rainbow-colored silverdeposit having a crystalline surface appearance. The 1 M solution wassimilar, with less of a crystalline effect. Under a low-poweredmicroscope, the surface appeared to be coated with silver. The 0.1 molarsample resulted in a very fine silver deposit on the polymer surface.The 0.01 solution deposited a barely-visible silver layer which had abluish hue.

I claim:
 1. A method for applying a silver coating to a poly(arylenesulfide) substrate, the method comprisingapplying to a surface of thepoly(arylene sulfide) substrate treating materials consistingessentially of an aqueous solution consisting essentially of AgNO₃ ; andheating the thus-treated surface to a temperature effective forproducing a silver coating on the surface of the poly(arylene sulfide)substrate.
 2. The method of claim 1 in which the surface is heated to atemperature of at least about 350° F.
 3. The method of claim 2 in whichthe surface is heated to a temperature of about 350° F.
 4. The method ofclaim 1 in which the AgNO₃ solution has a concentration within the rangeof about 0.01 to about 2 M silver nitrate.
 5. A method for applying asilver coating to a poly(arylene sulfide) substrate comprising the stepsofincreasing the surface wettability of a surface of the poly(arylenesulfide) substrate to be coated, and apply treating materials consistingessentially of an aqueous solution consisting essentially of AgNO₃ tothe surface at a temperature effective for causing the deposition of asilver coating onto the surface.
 6. The method of claim 5 in which thesurface is at a temperature of at least about 350° F.
 7. The method ofclaim 6 in which the surface is at a temperature of from about 350° F.to about 500° F.
 8. The method of claim 5 in which the surfacewettability of the surface to be coated is raised by contacting thesurface with a flame.
 9. The method of claim 5 in which the surfacewettability of the surface to be coated is raised by contacting thesurface with a methane flame for a time sufficient to eliminate beadingof a water drop on the surface.
 10. The method of claim 8 in which theaqueous AgNO₃ solution has a concentration of about 0.01 to about 2Msilver nitrate.
 11. The method of claim 8 in which the polymericsubstrate comprises poly(phenylene sulfide).
 12. An article ofmanufacture comprising poly(arylene sulfide) having a silver coatingapplied according to the method of claim 1
 13. An article of manufacturecomprising poly(arylene sulfide) and silver deposited on thepoly(arylene sulfide) according to the method of claim 5.